1. Field of the Invention
This invention relates in general to a compound semiconductor epitaxial wafer and a method for manufacturing it, and more particularly to a GaAsP epitaxial wafer and a method for manufacturing it.
2. The Prior Art
A light emitting diode is manufactured by using compound semiconductors of groups III-V such as GaP, GaAs, GaAsP and GaAlAs. Light is generated by feeding a forward direction current through the pn junction of said compound semiconductor. The wavelength of emitted light is different depending on the composition of the compound semiconductor and the impurities added therein.
A GaAsP (gallium phosphide arsenide) epitaxial wafer for a light emitting diode with a composition of GaAs.sub.1-x P.sub.x (0.45&lt;x&lt;1) emits light of the wavelength which varies according to the forbidden band energy gap of the semiconductor by changing the alloy composition (x) thereof. The light emitting diode emits red, orange or yellow light according to the values of the alloy composition (x) when x=0.50, x=0.65 or x=0.90, respectively. The light emitting diode with the aforementioned composition is of an indirect transition type. The constant composition layer is doped with nitrogen which becomes a light emitting center to increase the probability of radiative recombination and thus improve the light emitting efficiency.
As shown in FIG. 6, a conventional GaAsP epitaxial wafer is manufactured, for example, by: carrying out the epitaxial growth of an n-type GaAs.sub.1-x P.sub.x graded layer 23 in which the alloy composition is gradually increased from 0 and n-type GaAs.sub.1-x P.sub.x constant composition layers 24, 25 and 26 with constant alloy composition one after another on a GaP single crystal substrate 21, and then diffusing a p-type impurity from the surface of said constant composition layer 26 to form a pn junction between a p-type constant composition layer 26a which corresponds to a region in the layer 26 where the impurity was diffused and an n-type constant composition layer 26b which corresponds to a region in the layer 26 where the impurity was not diffused. The layer 25 is an increasing nitrogen concentration layer in which gradually more nitrogen is added and layer 26 is a constant nitrogen concentration layer.
Conventionally, the epitaxial growth of constant nitrogen concentration layer 26 was carried out while a relatively low concentration of nitrogen was added so that the initial light intensity would not decrease due to absorption of light by the layer.
The light emitting diode made from the GaAsP epitaxial wafer radiates light of a wavelength corresponding to the band gap of the constant nitrogen concentration layer 26 from the pn junction. Generally, a light emitting diode exhibits a degradation in which the light intensity decreases over the course of its operation life. It is known that a light emitting diode of 0.3 mm.times.0.3 mm square which has operated for 1,000 hours by a 30 mA current (current density 33 A/cm.sup.2) decreases in its light intensity down to 40-80% of the initial intensity immediately after it is just energized, though it depends on the operation environment. For example, in the case of a yellow light emitting diode with nitrogen concentration of 2.0.times.10.sup.18 cm.sup.-3, it degradates down to 40% of the initial light intensity after it has operated for 1,000 hours with a current density of 33 A/cm.sup.2 at 25.degree. C. Therefore, it is very important to reduce degradation of light intensity and improve the operation life in terms of the characteristics of a light emitting diode.